1. Technical Field
The present invention relates to a thermal management device useful for applications including cooling electronic components. More particularly, the present invention relates to a base heat spreader with a plurality of fins with improved characteristics for dissipating heat from electronic components such as semiconductor chips so that the components are reduced to a lower temperature state. The invention also includes a novel method for the removal of heat from components including electronic devices of a relatively small size.
2. Background Art
With the advancement of electronic devices being produced in increasingly smaller and smaller sizes coupled with the integration of multiple components into a single unit, cooling electronic devices have become a substantial industry. Specifically, in devices such as central processing units (CPUs) which contain integrated circuit-type electronics, the unit itself contains multiple components which produce heat and the performance of a CPU itself decreases unless sufficient thermal energy is removed from the unit. Semiconductors also suffer in that their performance deteriorates when the operating temperature increases to an undesired level around the semiconductor device.
The traditional method of cooling an electronic device is by using a fan to circulate air around the electronic device, and thus remove thermal energy. Unfortunately, the problems associated with fan-type technology are multiple, including a relatively low cooling efficiency, a bulky power source and a limited functional size. Furthermore, with the growing trend of devices which use electronic components becoming smaller and smaller, traditional cooling methods, including fans, are becoming much more limited as both a decrease in size and decrease in the acceptable weight of thermal management systems are necessitated by the industry. However, methods have developed to replace fan-type cooling devices which are both smaller and located in the immediate vicinity of the electronic device needing to be cooled. One purpose of the present invention is to provide a heat management device which can eliminate the use of fans, but more generally, it is to provide a more efficient heat sink for use in situations where heat sink performance is limited by convection from the fins themselves, allowing reduction of airflow requirements or lower thermal resistance for a given airflow.
One such type device is a heat pipe which has been demonstrated to remove heat from semiconductor chips. For example, Kroliczek, U.S. Pat. No. 6,382,309, discloses a loop heat pipe incorporating an liquid transfer medium having a wick that is liquid superheat tolerant and resistant to back conduction. The liquid heat pipe as disclosed in the '309 patent is a two-phase heat transfer system which includes a continuous loop in which both vapor and liquid always flow in the same direction. The device functions as to absorb heat through the evaporation of a liquid working fluid at an liquid transfer medium section of the liquid heat pipe and then, transports the vaporized fluid to a condenser section wherein the vaporized fluid is condensed. Furthermore, the use of a heat pipe as described in the '309 patent describes the latent heat of vaporization/condensation working fluid which allows for the transfer of relatively large quantities of thermal energy with small amounts of fluid and negligible temperature drops. Additionally, the '309 patent incorporates an improved wick which is resistant to back-conduction of heat energy, a problem associated with many traditional loop heat-type devices.
In Myer IV et al. (U.S. Pat. No. 5,642,776), an insulated heat pipe is claimed which includes a simple foil envelope. The patent claims metal foil sheets which are sealed at corresponding edges to encompass a wick comprised of a semi-rigid sheet of plastic foam including channels cut into the surface of the wick. Furthermore, the disclosed coolant for use in the loop heat pipe is water.
Other designs of thermal management systems have been developed to remove heat from small electronic devices, specifically devices for use in both computers and other high-end microelectronics.
In U.S. Pat. No. 6,988,315, Parish IV et al. describe a cooling apparatus having low profile extrusions and a method of manufacturing the cooling device. The patented invention is a device for circulating a heat transfer fluid including means for removing heat from the heat-transfer fluid. Furthermore, the Parish et al. patent is intended to be used to remove heat in applications including printed circuit boards of electronic components and also for use as general heat exchangers in applications where space is limited weight is critical.
Lim et al., U.S. Pat. No. 6,334,480, discloses a cooling device with micro-cooling fins. The patent claims a cooling device with micro-cooling fins which have an efficient heat-releasing structure. Essentially, the efficient heat-releasing structure includes a plurality of vibrating-type cooling fins extending above the heat exchanging device wherein the vibrating fins reduces the air space accumulated in a heat boundary layer which can hamper the performance of traditional cooling fins. Additionally, this patent claims that the use of the vibrating fins allows for an increased efficiency of removal of thermal energy while allowing for the invention to be used in increasingly smaller electronic devices.
U.S. Pat. No. 6,883,337 issued to Shyy et al., claims a thermal management device comprising a vapor compression cycle heat transfer device with a drop-wise condenser. Allegedly, the device is highly energy efficient and can be connected with similar type structures to form a row of thermal management devices for incorporation within small electronic devices such as chip packages. Additionally, the patent claims that the device is gravity insensitive and can provide a relatively high cooling rate for electronic instruments while being scaled to accommodate different types of applications.
In Lee (U.S. Pat. No. 6,698,502), a micro-cooling device is disclosed as having a high cooling efficiency which is neither influenced by gravity nor restricted in the position of installation. The device includes the coolant being stored inside a chamber, and also a heat-absorbing part wherein the liquid coolant can be vaporized to gaseous coolant for the transfer of thermal energy from the electronic device. The invention further includes interior micro channels which are designed so that the fluid coolant may move through capillary force and thus, not be constricted by limitations due to gravity. Additionally, the Lee patent claims that the circulation of the coolant in the cooling device is carried out without external driving forces, and instead is through the capillary phenomenon of surface tension of the liquid coolant within the invention. One method of inducing this capillary action is by use of a thin metal screen mesh providing for micro channels for the passage of the liquid coolant within the system.
Unfortunately, heat transfer devices produced by the prior art processes do not provide sufficient heat dissipation for many applications, or require extensive processing cost to construct the thermal management device. In addition, some of the aforementioned devices can be prone to failure, thus leading to excessive heat as heat transfer is significantly diminished.
In International Application No. WO 2006/073269, a heat transfer device and manufacturing methods thereof using a hydrophilic wick are disclosed. This device precludes many of the aforementioned problems of the loop heat pipes and chambers utilizing capillary action by incorporating a device with both the hydrophilic wick and a support structure. The device is intended to be a flat panel device using a water coolant which efficiently eliminates the possibility of drying out while also providing efficient cooling through the vaporization and condensation of the water coolant.
One disadvantage of the device of International Application No. WO 2006/073269 is that its design is for a flat panel heat exchanging device and does not incorporate the use of fins which can significantly improve the performance of the device. Furthermore, the device does not include sufficient methods to reduce localized hot spots produced by an electronic device and does not adequately transfer thermal energy along the device's entire surface.
An additional method of managing thermal energy is through graphite-based components which offer thermal conductivity comparable with or better than copper or aluminum but at a fraction of the weight while providing significantly greater design flexibility. Graphite-based thermal management products take advantage of the highly directional properties of graphite to move heat away from electronic components while having thermal conductivities substantially higher than typical aluminum alloys used for heat management. Furthermore, graphite is anisotropic making it more suitable for channeling heat in a preferred direction.
What is desired, therefore, is an improved thermal management device for use with a wide variety of electronic devices including electronic devices of an extremely small size wherein the thermal management device has improved properties. Furthermore, what is desired is heat sink with improved efficiency for use when the dissipation of thermal energy is limited by convection, where the heat sink will allow for more efficient forced convection, including a lowering of the thermal resistance for a given airflow and possibly the elimination of fan-type components. Indeed, a combination of characteristics including the reduction of localized hot spots of thermal energy and the relatively even spreading of thermal energy coupled with the ability to be gravity insensitive while providing sufficient cooling have been found to be needed for use of thermal management devices in small scale electronic devices. Also desired is a method which is improved for removing heat from electronic devices.